The following correction was printed in the Observer's For the record column, Sunday 9 August 2009

We should clarify that the report from the Commons select committee on innovation, universities, science and skills applies to England only. Higher education policy for Northern Ireland, Scotland and Wales is covered by their respective parliaments and assemblies. And Gillian Evans, quoted in the piece, is emeritus professor of medieval theology at Cambridge, not Oxford.

Universities were yesterday embroiled in a furious row over dumbing down after a parliamentary inquiry revealed the number of first-class degrees had almost doubled in a decade. Amid the war of words, senior Tories vowed to publish data that they claimed would reveal the true value of degrees.

Different universities demand "different levels of effort" from students to get similar degrees, according to the report from the commons select committee on innovation, universities and skills, suggesting that top grades from some colleges were not worth the same as others.

It calls for the watchdog overseeing standards in universities to be radically overhauled or scrapped and new guides set for degree marking, noting that while 53% of students achieved a first or 2:1 in 1997, that had risen by last year to 61%.

Universities claimed that standards must be high because colleges remained popular with overseas students, but the committee said it was "absurd and disreputable" to justify academic prowess in that way. Phil Willis, chair of the committee, said it was "extremely concerned that inconsistency in standards is rife".

But Willis said the universities had been "defensive" and complacent, and had failed to address concerns about grade inflation. "We have seen the number of first-class degrees double, yet when we ask 'why is that?', you are faced with 'why are you asking us that?'," he said.

Meanwhile, the Conservatives pledged last night to open universities to tougher scrutiny by publishing data next month that details what happens to students' job prospects and salaries when they leave individual institutions, exposing for the first time how college choice affects lifetime earnings.

The move potentially threatens those universities at the bottom of league tables, whose students may get very minimal returns. "Young people will focus much more on this if we get really crunchy figures - if you knew you could earn, say, £400,000 more over a lifetime by doing engineering at a Russell Group university and £50,000 if you do English at a lesser one," said David Willetts, the higher education spokesman.

The committee's report comes as thousands of teenagers await A-level results this month which may decide their university careers. It concludes there is too little research on degree standards to be sure what caused the rise, but cites a study from Lancaster University identifying better teaching, keener students and, critically, the pressure to do well in league tables as likely factors.

The percentage of firsts awarded rose from 7.7% in 1996-1997 to 13.3% last year, while 2:1s were up from 44.5% to 48.1%. The rise was across all subjects and sharpest in elite Russell Group universities until 2002, when it grew more widespread. The report concluded that universities could not simply argue grades had risen in line with A-level performance because "if A-level grades have inflated unjustifiably, and there are many who think they have, then so must degree classes".

Gillian Evans, a lecturer in mediaeval theology at Oxford University and an expert in university regulation, attributed the rise to universities' desire to move up published league tables.

"I am quite sure the reason proportions have gone up is exactly the same as the reasons A-levels have gone up: it's straightforward grade inflation, chasing a place in league tables," she said.

However, Warwick insisted the increase in grades was "not particularly marked" and the proposed reforms were "a sledgehammer to crack a nut". Universities were already moving away from the honours degree system towards a US-style "report card" system reflecting skills gained and academic achievements.

She added: "It is ironic that they were focussing on the hierarchy of firsts and 2:1s when ... we have regarded that as rather a blunt instrument."

Wes Streeting, of the National Union of Students, said the report raised important questions about value for money: "We find it astonishing that universities continue to demand ever higher fees without showing how they make a difference to the people who pay for them."

Research suggests even students gaining good degrees from some new universities may struggle. A 1997 study from the University of Kent suggested a third of graduates ended up in jobs not requiring degrees, with students from ex-polytechnics faring worst.

The Tories have worked with a leading Kent academic to compile information usually withheld by universities about where their graduates end up working. It will be published online from next month.

Lord Mandelson, the cabinet minister for higher education, said last night that he did not "recognise the committee's description" of universities.